1. Field of the Invention
The present invention relates to a magnetoresistive element, a magnetoresistive random access memory (MRAM), and an electronic card and electronic device using the same.
2. Description of the Related Art
A magnetoresistive random access memory (MRAM) using a tunneling magnetoresistive (TMR) effect is characterized in storing data in accordance with the magnetization state of a magnetic tunnel junction (MTJ) element. In order to get the magnetoresistive random access memory into practical use, various kinds of techniques have been proposed.
For example, a yoke wiring structure is proposed to reduce a write current. As the structure of an MTJ element, a structure using a perpendicular magnetic film made of a GdFe alloy (e.g., nonpatent reference 1) and a layered structure using a perpendicular magnetic film (e.g., nonpatent reference 2) are proposed. They basically employ a field write scheme of reversing the magnetization direction of a magnetic layer by using a magnetic field generated by a current. When the current is large, a large magnetic field can be generated. However, as microfabrication progresses, the current that can be supplied to the wiring is limited. By realizing the reduction of the distance between the wiring and the magnetic layer and/or the yoke structure to concentrate the generated magnetic field, the current value necessary for reversing the magnetic material can be reduced. However, since the magnetic field necessary for magnetization reversal of the magnetic material increases along with the progress of microfabrication, it is very difficult to simultaneously implement both current reduction and microfabrication. The magnetic field necessary for magnetization reversal of the magnetic material is increased by microfabrication because a magnetic energy to overcome thermal agitation is required. The magnetic energy can be made high by increasing the magnetic anisotropy energy density and the volume of the magnetic material. Since the volume is decreased by microfabrication, a shape magnetic anisotropy energy or magnetocrystalline anisotropy energy is used in general. However, since an increase in magnetic energy of the magnetic material leads to an increase in reversal field, as described above, it is very hard to implement both current reduction and microfabrication simultaneously. Patent reference 1 proposes a yoke structure of completely closed magnetic circuit type which introduces a perpendicular magnetic film with a high magnetocrystalline anisotropy energy and has an ultimately high current magnetic field generation efficiency. Since this yoke structure becomes large relative to the magnetic element, the cell area becomes relatively large so all the microfabrication, current reduction, and cell area reduction cannot be satisfied.
In recent years, magnetization reversal by a spin polarized current is theoretically predicted and also confirmed by experiments. A magnetoresistive random access memory using a spin polarized current is proposed (e.g., nonpatent reference 3). According to this scheme, the magnetization of a magnetic material can be reversed only by flowing a spin polarized current to the magnetic material. When the volume of the magnetic material is small, the amount of spin-polarized electrons to be injected can also be small. This scheme is therefore expected to implement both microfabrication and current reduction. In addition, since no magnetic field generated by a current is used, no yoke structure to increase the magnetic field is necessary, and the cell area can be reduced. In the magnetization reversal scheme using a spin polarized current, however, the problem of thermal agitation still rises along with the progress of microfabrication. To overcome thermal agitation, the magnetic anisotropy energy density must be increased, as described above. An in-plane magnetization structure mainly examined until now generally uses shape magnetic anisotropy. In this case, magnetic anisotropy is ensured by using the shape. For this reason, the reversal current is sensitive to the cell shape, and a variation in reversal current increases as microfabrication progresses. Since the aspect ratio of an MTJ cell must be at least 1.5, the cell size also increases. When an in-plane magnetization structure using not shape magnetic anisotropy but magnetocrystalline anisotropy uses a material having a high magnetocrystalline anisotropy energy density (e.g., a Co—Cr alloy material used in a hard disk medium), the easy axis is largely dispersed in plane. Hence, reduction of the magnetoresistive (MR) effect and incoherent precession are induced, resulting in an increase in reversal current.
There are reported several examples of a perpendicular magnetic MTJ structure, as described above, although no detailed means to form a large-scale array by the write scheme using a spin polarized current has been proposed.
As described above, the conventional magnetoresistive random access memory preferably simultaneously reduces the write current, overcomes the thermal agitation, and reduces the cell area. However, this is very difficult in the write scheme using a magnetic field generated by a current. Even in the conventional write scheme using a spin polarized current, no detailed means is proposed to overcome the thermal agitation that becomes apparent along with the progress of microfabrication.    [Patent Reference 1] Jpn. Pat. Appln. KOKAI Publication No. 2005-19464    [Nonpatent Reference 1] Ikeda, et al., “GMR film and TMR film using GdFe alloy perpendicular magnetic film”, JOURNAL OF MAGNETIC SOCIETY OF JAPAN, Vol. 24, No. 4-2, 2000, pp. 563-566    [Nonpatent Reference 2] N. Nishimura, et al., “Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory”, JOURNAL OF APPLIED PHYSICS, Vol. 91, No. 8, 15 April, 2002    [Nonpatent Reference 3] J. C. Slonczewski, et al., “Current-driven excitation of magnetic multilayers”, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, Vol. 159, No. 1-2, L1-7, 1996    [Nonpatent Reference 4] K. Yagami, et al., “Low-current spin-transfer switching and its thermal durability in a low-saturation-magnetization nanomagnet”, APPLIED PHYSICS LETTERS, Vol. 85, No. 23, pp. 5634-5636, 2004